Polyester film for magnetic recording media, process for preparation thereof and magnetic recording medium

ABSTRACT

A polyester film for magnetic recording media, a process for preparation thereof and a magnetic recording medium comprising the film having formed on the surface of a ferromagnetic metallic thin membrane, said polyester film being one wherein on the surface of one side or both sides of the film, 
     (i) fine protuberances having an average height of 0.15 μm or less exist at a rate of 1.0×10 4  to 1.0×10 8  per mm 2 , 
     (ii) the fine protuberances are composed of fine particles whose surface is coated with a binder resin, 
     (iii) the fine protuberances are stuck on the film surface by the binder resin, 
     (iv) the binder resin does not substantially exist on the film surface other than in the vicinity of the film surface where the fine protuberances exist, and 
     (v) the average surface roughness (Ra) of the film surface is in the range of 0.001 to 0.01 μm.

This application is a continuation of now abandoned application Ser. No.07/903,323, filed Jun. 24, 1992.

DETAILED DESCRIPTION OF THE INVENTION

1. Industrially Applicable Field

This invention relates to a polyester film for magnetic recording media,a process for preparation thereof and a magnetic recording medium. Morespecifically, this invention relates to a magnetic recording mediumexcellent in running properties, electromagnetic conversioncharacteristics, and durability in preservation, and a polyester filmused therefor and a process for preparation of the film.

2. Prior Art

As high density magnetic recording media, ferromagnetic metallic thinmembrane magnetic recording media are known which each comprise anonmagnetic substrate having formed thereon a ferromagnetic metallicthin membrane by a physical deposition method such as vacuum depositionor sputtering or a plating method. For example, there are known amagnetic tape comprising a film having vapor deposited Co thereon(Japanese Laid-Open Patent Publication No. 147010/1979), a verticalmagnetic recording medium wherein a Co-Cr alloy is used (JapaneseLaid-Open Patent Publication No. 134706/1977), etc. Such a metallic thinmembrane formed by a thin membrane formation means such as vapordeposition, sputtering or ion plating has an advantage that itsthickness is as thin as 1.5 μm or less, and nevertheless it hasperformances equal to or more than those of a coating type magneticrecording medium (a magnetic recording medium comprising a nonmagneticsubstrate having applied thereon magnetic substance powder mixed into anorganic high molecular binder) wherein the thickness of the magneticrecording layer is 3 μm or more.

By the way, it has been considered that the coercive force (Hc) as astatic characteristic or magnetic characteristics such as therectangular ratio of hysteresis loop of magnetic recording media do notmuch depend on the surface state of the nonmagnetic substrates used.Based on this conception, a Co-Cr multilayer structure by vacuumdeposition is disclosed in U.S. Pat. No. 3,787,327.

However, in case of a metallic thin membrane type magnetic recordingmedium, it has a disadvantage that since the thickness of the metallicthin membrane formed on the nonmagnetic substrate surface is extremelythin, the surface state (surface irregularities) of the nonmagneticsubstrate (film) is expressed, as it is, as irregularities of thesurface of the magnetic recording layer and becomes a cause of noise.

In view of noise, it is preferred that the surface state of thenonmagnetic substrate (film) be as smooth as possible. On the otherhand, in view of handling such as winding or unwinding of the base film,if the film surface is smooth, slip properties between film and film arebad, blocking phenomenon occurs and thus such a magnetic recordingmedium cannot become a product, and therefore it is required that thebase film surface is rough. Thus, in view of electromagnetic conversioncharacteristics, smoothness of the surface of the non-magnetic substrateis required, and on the other hand, in view of handling, roughness ofthe surface is required. Therefore, it is required to satisfy theantinomic properties of both of them at the same time.

Further, as for metallic thin membrane magnetic recording media, thereis the problem of running properties of the metallic thin membrane sideas an important problem at the time of actual use. The usual case of acoating type magnetic recording medium comprising a base film havingapplied thereon magnetic substance powder mixed into an organic highmolecular binder, the running properties of the magnetic side can beimproved by dispersing a slip agent in the binder. However, in the caseof a metallic thin membrane magnetic recording medium, such a solutionmethod cannot be used, it is very difficult to maintain runningproperties stable, and particularly, it had disadvantages, e.g., thatrunning properties at the time of high temperature and high humidity arepoor.

In order to improve this disadvantage, it is proposed in Japanese PatentPublication No. 30105/1987 to form on the film surface protuberancescomposed of fine particles, a water soluble resin and a silane couplingagent, and it is proposed in Japanese Patent Publication No. 30106/1987and Japanese Laid-Open Patent Publication No. 229316/1984 to form on thefilm surface protuberances composed of fine particles and a watersoluble resin. However, in all the products, the fine particles exist inthe trapezoidal protuberances of the water soluble resins, and do notuniformly exist on the film surface.

In Japanese Laid-Open Patent Publication No. 63150/1985, it is proposedto form two kinds of protuberances on the film surface, larger andsmaller. However, as is seen from the examples, these protuberances aremainly formed by fine destruction of the high molecular membrane, andthus there is a problem in the uniformity and reproducibility of thefilm surface.

On the other hand, it has been proposed, as described below, to formparticulate protuberances on the film surface using fine particles asnuclei and a resin as a binder.

(i) Japanese Laid-Open Patent Publication No. 30231/1984

A magnetic recording medium which comprises a plastic film whereinparticulate resin protuberances each containing as a nucleus a fineparticle of a thermoplastic resin and having a particle height of 30 to500 Å were made to exist on the surface in a number of 1,000 per mm² ormore, having formed on the surface of the plastic film a ferromagneticmetallic thin membrane having made a slip agent exist thereon.

(ii) Japanese Laid-Open Patent Publication No. 48825/1984

A magnetic recording medium which comprises a plastic film substratehaving provided thereon an undercoat layer containing 1000/mm² or moreof particulate protuberances which each comprises a silica fine particleas a nucleus and a resin as a binder and have a particle height of 30 to500 Å, having formed on the undercoat layer a ferromagnetic metallicthin membrane having further made a slip agent exist on the metallicthin membrane.

(iii) Japanese Laid-Open Patent Publication No. 92428/1984

A magnetic recording medium which comprises a plastic film havingprovided on the surface thereof an undercoat layer containingparticulate protuberances which comprise as nuclei a mixture of twokinds of fine particles having average heights of 30 to 200 Å and 100 to600 Å and a resin as a binder, having formed on the undercoat layer aferromagnetic metallic thin membrane.

(iv) European Patent No. 0077549B1, Japanese Laid-Open PatentPublication No. 68227/1983 and Japanese Laid-Open Patent Publication No.100221/1983

A magnetic recording medium, comprising a plastic film over the surfaceor surfaces of which a thin ferromagnetic film is formed as a magneticlayer, said surface or surfaces being imparted with protuberant orrumple-or worm-like nodules, characterized in that into said protuberantor rumple-or worm-like nodules finely divided particles of the diameterbetween 0.01 and 0.2 μm are dispersed.

(v) U.S. Pat. No. 4,578,729

A magnetic recording medium comprising a thin ferromagnetic metal filmformed as a magnetic layer; and strip protrusions having a heightbetween 100 and 2,000 Å distributed over the surface of said magneticlayer at an average density ranging from 10⁴ to 10⁶ protrusions per mm²,and the ratio between the length and the height of said protrusions isless than 10:1.

(vi) U.S. Pat. No. 4,688,130

A perpendicular magnetic recording medium comprising:

a substrate having 5×10⁵ -5×10⁹ fine projections per mm² formed on andprojecting from the surface thereof, said projections having heights inthe range of 0.01-10 μm and diameters in the range of 0.01-10 μm; and

fine particles of a ferromagnetic material deposited onto each of theprojections uniformly in a columnar structure, said columnar structureextending perpendicular to said surface and being substantially isolatedfrom each other with their axes of easy magnetization also extendingperpendicularly to said surface.

(vii) U.S. Pat. No. 4,645,703

A magnetic recording medium comprising: a substrate of non-ferromagneticcharacteristic, and a ferromagnetic thin film, formed on said substrate,characterized by protrusions of mountain-shape having 50-600 Å heightsand distributed with numbers of 1×10⁵ -1×10⁸ protrusions per 1 mm² on asurface of said ferromagnetic thin film, and

a lubricant layer formed on said ferromagnetic thin film.

(viii) U.S. Pat. No. 4,540,618

A magnetic recording medium comprising; a substrate of plastic film anundercoating layer having 1000/mm² or more minute granular protrusionsof 30 to 500 Å in height provided that the lateral spread is in therange of 1:1 to 20:1, said undercoating layer being formed as a coatingon the face of said substrate,

a ferromagnetic thin film formed on said undercoating layer, and

a lubrication layer formed on said ferromagnetic thin film wherein saidgranular protrusions comprise nuclei of at least one substance selectedfrom the group consisting of a thermoplastic resin, a crosslinked resin,carbon, an oxide of a metal and the hydrolysis product of a metalalkoxide.

(ix) U.S. Pat. No. 4,564,549

A magnetic recording medium characterized in that a thin film offerromagnetic metal containing oxygen is formed over the surface of anelongated polyester film which is coated with worm-like or particle-likeprojections comprising a cured high-molecular weight compound having asurface roughness, R_(max), of 0.003 to 0.1 micrometers and wherein theback surface of said film also has worm-like or particle-likeprojections comprising a cured high-molecular weight compound and isfurther formed with a thin film of a high-molecular weight compoundcontaining a lubricant.

(x) U.S. Pat. No. 4,732,814

A biaxially stretched polyester film having a multiplicity of elongatedand directionally arranged ultrafine protrusions on the surface thereof,wherein said protrusions have a height of 50 to 300 angstroms and awidth of 100 to 300 angstroms and 1 to 20 protrusions are present per μmof the surface length as observed on a cross-section perpendicular tothe direction of the arrangement of the protrusions, said protrusionsbeing an integral part of the polyester film and tightly adherentthereto and being dyeable with osmic acid, and said film having a staticfriction coefficient (μs) of 0.3 to 3.0.

(xi) U.S. Pat. No. 4,735,848

A magnetic recording medium comprising a non-magnetic base, a thin filmmetal magnetic layer formed on said non-magnetic base, a first layercontaining a rust preventive formed on said magnetic layer and a secondlayer containing a lubricant formed on said first layer, said secondlayer containing a carboxylic acid perfluoroalkyl ester represented bythe following general formula:

    RCOO(CH.sub.2).sub.j C.sub.k F.sub.2k+1

where R is a hydrocarbon residue, j is an integer and may be 0, and k isan integer not less than 4, said second layer also containing an extremepressure agent selected from the group consisting of phosphoruscontaining, sulfur containing, halogen containing, and organo metallicextreme pressure agents, and composite type extreme pressure agents.

(xii) U.S. Pat. No. 4,135,031

A magnetic recording substance comprising a non-magnetic support filmhaving a magnetic recording layer on one side and a back coatingcomprising a thin layer of a resinous binder having dispersed therein amixture of inorganic powders comprising as a major component aninorganic powder having grain size of 0.02-5 microns and as a minorcomponent an inorganic powder having a grain size of 2-40 microns, saidminor component forming with said major component and binder on saidfilm a coating having spike grains extending outwardly from thenon-magnetic surface to a height of 0.8-5 microns at a pitch of at least200 microns to yield a tape having a coefficient of friction of at least0.25.

However, in the above techniques so far known, there is a problem thateven if the size and volume of the fine particles can be controlled,formation of protuberances of the binder resin itself cannot be avoided,in addition to formation of particulate protuberances, on the filmsurface, and moreover, as for formation of protuberances of the resinitself, control of their size and volume is extremely difficult.

Problems to be Solved by the Invention

The object of this invention lies in providing a magnetic recordingmedium free of the aforementioned drawbacks of the usual techniques andexcellent in running properties, electromagnetic conversioncharacteristics, etc., particularly a metallic thin membrane magneticrecording medium, a polyester film used therefor and a process forpreparation thereof.

Specifically, the object of this invention lies in providing a polyesterfilm wherein, on the film surface, the number of protuberances based onthe binder resin itself are as small as possible and, in other wordssubstantially only protuberances based on the fine particles are formed;a method for preparation thereof; and a magnetic recording medium fromthe film.

Means for Solving the Problems

According to research by the present inventors, it was revealed that theabove object of this invention can be attained by

a polyester film for magnetic recording media wherein on the surface ofone side or both sides of the film,

(i) fine protuberances having an average height of 0.15 μm or less existat a rate of 1.0×10⁴ to 1.0×10⁸ per mm²,

(ii) the fine protuberances are composed of fine particles whose surfaceis coated with a binder resin,

(iii) the fine protuberances are stuck on the film surface by the binderresin,

(iv) the binder resin does not substantially exist on the film surfaceother than in the vicinity of the film surface where the fineprotuberances exist, and

(v) the average surface roughness (Ra) of the film surface is in therange of 0.001 to 0.01 μm,

and can further be attained by a magnetic recording medium whichcomprises a polyester film having the above characteristics havingformed on its surface a ferromagnetic metallic thin membrane.

Hereafter, this invention is described in more detail.

In this invention, the polyester forming the polyester film is a linearsaturated polyester synthesized from an aromatic dibasic acid or anester forming derivative thereof with a diol or an ester formingderivative thereof. Specific examples of such polyesters arepolyethylene terephthalate, polyethylene isophthalate, polybutyleneterephthalate, poly(1,4-cyclohexylenedimethylene terephthalate),polyethylene-2,6-naphthalenedicarboxylate, etc., and further includetheir copolymers or blends of them with other resins in a small ratio,etc. Among these polyesters, polyethylene terephthalate andpolyethylene-2,6-naphthalenedicarboxylate, especially polyethyleneterephthalate are preferred for accomplishment of the object of thisinvention.

According to conventional methods, the above polyester is melt-extruded,biaxially stretched and oriented, and heat-set to obtain a film. Thebiaxial stretching can be carried out by a biaxial stretching methodsuch as, for example, a consecutive biaxial stretching method or asimultaneous biaxial stretching method. This biaxially orientedpolyester film usually has such a characteristic of orientation ofcrystals that its heat of fusion measured, using a differential scanningcalorimeter, under a nitrogen atmosphere at a heating rate of 10° C./minis 4 cal/g or more. The thickness of the film after orientation bystretching is in the range of generally 3 to 100 μm, preferably 4 to 50μm.

The polyester film of this invention has on the surface of one side orboth sides fine protuberances having an average height of 0.15 μm orless, preferably 0.1 μm or less and existing in a number of 1.0×10⁴ to1.0×10⁸ /mm², preferably 1.0×10⁵ to 8.0×10⁷ /mm², and has a surfaceroughness Ra of 0.001 to 0.1 μm, preferably 0.001 to 0.007 μm. The fineprotuberances are fine particles coated with a resin as a binder, andstuck on the film surface with the binder resin. In the polyester filmof this invention, the above fine protuberances based on the fineparticles exist on its surface, and fine protuberances formed only ofthe binder resin do not substantially exist, and the binder resin doesnot substantially exist except in the vicinity of the film surface wherethe fine protuberances based on the fine particles exist. Thus, thepolyester film of this invention has a characteristic that, on thesurface thereof, there are formed protuberances substantially consistingof the fine protuberances comprising the fine particles coated with thebinder resin, and other protuberances and resins do not substantiallyexist.

Since substantially all of the fine protuberances on the surface of thepolyester film of this invention derive from fine particles coated withthe binder resin, the size, shape and number of the fine protuberancescan be controlled accurately, and therefore, the polyester film can besaid to be a film having a surface extremely excellent in uniformity.The number of the fine protuberances on the film surface can be countedusing a scanning type electron microscope, and is a value obtained bycounting the number of protuberances having a height from the surface of0.005 μm or more.

Fine particles existing in the above fine protuberances in thisinvention can be any of fine particles of organic substances such aspolystyrene, polymethyl methacrylate, a methyl methacrylate copolymer, acrosslinked methyl methacrylate copolymer, polytetrafluoroethylene,polyvinilidene fluoride, polyacrylonitrile and a benzoguanamine resin;and fine particles of inorganic substances such as silica, alumina,titanium dioxide, kaolin, talc, graphite, calcium carbonate, feldspar,molybdenum disulfide, carbon black and barium sulfate. Preferably, thesefine particles have an average particle size of 0.1 μm or less. Thesefine particles have, particularly preferably, an average particle sizein the range of 0.003 to 0.06 μm, and it is desirable that the change ofthe particle size is small. For example, it is preferred that theparticle size is such that the value of relative standard deviationrepresented by the following equation is 0.5 or less, more preferably0.4 or less, particularly preferably 0.3 or less. ##EQU1##

wherein Di=circle-equivalent diameter of each particle ##EQU2##

As the above fine particles, monodispersed ones are used in view of itspurpose. Aggregates are not suitable because control of its size isdifficult.

As resins (binders) used for covering the above fine particles andsticking the fine protuberances on the surface of the polyester film,there can be exemplified alkyd resins, unsaturated polyester resins,saturated polyester resins, phenol resins, epoxy resins, amino resins,polyurethane resins, vinyl acetate resins, vinyl chloride-vinyl acetatecopolymer resins, acrylic resins, acrylic-polyester resins, etc. Thesemay be homopolymers or copolymers, or mixtures. Preferred among them areacrylic resins, saturated polyester resins and acrylic-polyester resins.These preferred resins are described in more detail below.

The above "acrylic resins" are those prepared from combinations ofmonomers, for example, acrylic esters (as alcohol residues, there can beexemplified a methyl group, an ethyl group, a n-propyl group, anisopropyl group, a n-butyl group, an isobutyl group, a t-butyl group, a2-ethylhexyl group, a cyclohexyl group, a phenyl group, a benzyl group,a phenylethyl group, etc.); methacrylic esters (alcohol residues are thesame as above); hydroxyl-containing monomers such as 2-hydroxyethylacrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl acrylate and2-hydroxypropyl methacrylate; amido group-containing monomers such asacrylamide, methacrylamide, N-methylmethacrylamide, N-methylacrylamide,N-methylolacrylamide, N-methylolmethacrylamide,N,N-dimethylolacrylamide, N-methoxymethylacrylamide,N-methoxymethylmethacrylamide and N-phenylacrylamide; aminogroup-containing monomers such as N,N-diethylaminoethyl acrylate andN,N-diethylaminoethyl methacrylate; epoxy group-containing monomers suchas glycidyl acrylate, glycidyl methacrylate and allyl glycidyl ether;monomers containing a sulfonic acid group or a salt thereof such asstyrenesulfonic acid and vinylsulfonic acid and their salts (e.g.,sodium salts, potassium salts, ammonium salts, etc.); monomerscontaining carboxyl group(s) or a salt thereof such as crotonic acid,itaconic acid, acrylic acid, maleic acid and fumaric acid and theirsalts (e.g., sodium salts, potassium salts, ammonium salts, etc.);anhydride-containing monomers such as maleic anhydride and itaconicanhydride; and further vinyl isocyanate, allyl isocyanate, styrene,vinyl methyl ether, vinyl ethyl ether, vinyltris(alkoxy)silanes,alkylmaleic acid monoesters, alkylfumaric acid monoesters,acrylonitrile, methacrylonitrile, alkylitaconic acid monoesters,vinylidene chloride, vinyl acetate and vinyl chloride. Preferred amongthem are those containing as a component a (meth)acrylic monomer such asan acrylic acid derivative or a methacrylic acid derivative in a ratioof 50 mol % or more, and particularly preferred are those containingmethyl methacrylate as a component.

Further, as acid components constituting the above "saturated polyesterresins", there can be exemplified polyvalent carboxylic acids such as,for example, terephthalic acid, isophthalic acid, phthalic acid,1,4-cyclohexanedicarboxylic acid, 2,6-naphthlenedicarboxylic acid,4,4'-diphenyldicarboxylic acid, adipic acid, sebacic acid,dodecanedicarboxylic acid, succinic acid, sodium isophthalicacid-5-sulfonate, potassium terephthalic acid-2-sulfonate, trimelliticacid, trimesic acid, trimellitic anhydride, phthalic anhydride,p-hydroxybenzoic acid and monopotassium trimellitate. Further, ashydroxy compound components, there can be exemplified polyvalent hydroxycompounds such as, for example, ethylene glycol, propylene glycol,1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, neopentyl glycol,1,4-cyclohexanedimethanol, p-xylylene glycol, ethylene oxide adducts ofbisphenol A, diethylene glycol, triethylene glycol, dimethylolpropionicacid, glycerol, trimethylolpropane, sodium dimethylolethylsulfonate andpotassium dimethylolpropionate. Polyester resins can be prepared fromthese compounds by a conventional method.

Further, the above "acrylic-polyester resins" included the forms ofacrylic-modified polyester resins and polyester-modified acrylic resins,are those wherein an acrylic resin component and a polyester resincomponent are mutually bound, and include, for example, those of a grafttype, those of a block type, etc. An acrylic-polyester resin can, forexample, be prepared by binding a radical initiator to both ends of apolyester resin and carrying out polymerization of an acrylic monomer,by binding a radical initiator to the side chains of a polyester resinand carrying out polymerization of an acrylic monomer, or by binding ahydroxyl group to the side chains of an acrylic resin and reacting itwith a polyester having an isocyanato group or a carboxyl group to givea comb-shaped polymer.

As the aforesaid binder resin is preferred one having good adhesion tothe polyester film and capable of softening at a temperature lower thanthe melting point of the polyester forming the film. Particularlypreferred is one having such adhesion and softening point that fineprotuberances stuck on the surface of the polyester film are formed inthe drying step or heat-setting step in the later-described process forpreparation of the polyester film of this invention. For example, it ispreferred that the softening point is in the range of 80° to 240° C.

As a method to coat the surface of the fine particles with the resin(binder), there can be used a method to carry out coating with thepolymer by an emulsion polymerization or dispersion polymerizationmethod using the fine particles as nuclei, a method to add dropwise thefine particles into a suitable polymer solution or suspension to coatthe surface of the fine particles, or the like. Preferred among them isthe method of carrying out polymer coating by emulsion polymerization ordispersion polymerization using the fine particles as nuclei, in orderto avoid formation of protuberances based only on the resin on the filmsurface. Resin-coated fine particles obtained by these methods are usedfor further steps after separation from the component consisting of theresin alone.

Preferably, the thus obtained fine particles coated with the resin as abinder have an average particle size of 0.15 μm or less. It is alsopreferred that the average particle size of the fine particles beforecoating with the resin is 0.1 μm or less.

According to research by the present inventors, it was found that theabove-mentioned polyester film of this invention having fineprotuberances on the surface can be prepared by applying onto thesurface of one side or both sides of the polyester film a dispersionobtained by uniformly dissolving in a readily volatile liquid mediumfine particles whose surface was coated with a binder resin; drying theresultant film, and then, if necessary, subjecting the film tostretching treatment and heat-setting treatment.

As a method to form fine protuberances on at least one surface of apolyester film in this invention, there can be adopted a method to applyonto the surface of the polyester film in a step for preparation of apolyester film a dispersion containing fine particles coated with abinder resin (coating liquid), preferably a water-dispersion, and thendry the film to stick the fine particles thereon; or a method to applyonto a biaxially oriented polyester film a dispersion containing fineparticles coated with a binder resin (coating liquid), and then dry thefilm to stick the fine particles thereon; or the like, but the formermethod is preferred. It causes no inconvenience to add a surfactant intosuch a dispersion in order to make the coating easier.

Preferably, the above application of the dispersion in a step forpreparation of a polyester film is made onto the surface of thepolyester film before completion of orientation of crystals. Hereinexamples of the polyester film before completion of orientation ofcrystals include an unstretched film obtained by hot-melt extruding apolyester simply into film; a monoaxially stretched film wherein theunstretched film is oriented in any one direction of the longitudinaldirection and the transverse direction; a biaxially stretched film suchthat although it is stretched in the biaxial directions, stretching inat least one of the directions is a low percent of stretching and thusfurther orientation with stretching at that direction is required (abiaxially stretched film before completion of orientation of crystals byfinal re-stretching in the longitudinal and/or transverse directions);etc.

The polyester film of this invention is, preferably, prepared by aso-called in-line coating method, namely by applying the abovedispersion onto a polyester film unstretched or stretched in at leastmonoaxial direction before completion of orientation of crystals, andthereafter subjecting the film to longitudinal stretching and/ortransverse stretching and heat-setting. In order to make possible smoothapplication of the dispersion onto the surface of the polyester filmbefore completion of orientation of crystals, it is preferred either tosubject the film surface to corona discharge treatment as a preliminarytreatment or to add into the dispersion a surfactant chemically inertthereto. Such a surfactant can decrease the surface tension to 40dyne/cm or less and thus promotes wetting of the polyester film, andthere can, for example, be mentioned anionic and nonionic surfactantssuch as polyoxyethylene aklylphenyl ethers, polyoxyethylene fatty acidesters, sorbitan fatty acid esters, glycerol fatty acid esters, fattyacid metallic soaps, alkyl sulfates, salts of alkylsulfonic acids andsalts of alkyl sulfosuccinates. Further, it is possible to mix into thedispersion other additives such as, for example, an antistatic agent, anultraviolet absorber and a lubricant in such a range not to lose theeffects of this invention.

In the process of this invention, the concentration of solid componentsin the dispersion, particularly in the water-dispersion is usually 30weight % or less, preferably 15 weight % or less.

Further, the liquid medium forming the dispersion is, preferably,readily volatile, and its boiling point is 30° to 240° C., preferably50° to 170° C. Further, it is preferred that the liquid medium does notdissolved the binder resin or the polyester or sparingly dissolves it.As liquid media, aqueous media are preferred, and for example, there canbe mentioned water, methyl alcohol, ethyl alcohol, isopropyl alcohol,ethylene glycol, 1,2-propylene glycol, diethylamine, triethylamine,n-butylamine, acetone, methyl ethyl ketone, methyl isobutyl ketone,cyclohexanone, ethyl ether, methyl acetate, ethyl acetate, methylformate, tetrahydrofuran, butylcellosolve, dioxane, dioxolan, xylene,etc.

As a method to apply the dispersion onto the surface of the polyesterfilm, a method known per se is applied. For example, a roll coatingmethod, a gravure coating method, a roll brush method, a spray coatingmethod, an air-knife coating method, a dipping method, a curtain coatingmethod, etc. may be applied singly or in combination.

Thus, a film can be obtained wherein fine protuberances exist on thesurface thereof in a rate of 1.0×10⁴ to 1.0×10⁸ per mm², preferably1.0×10⁵ to 8.0×10⁷ per mm² and which has a surface roughness Ra of 0.001to 0.01 μm, preferably 0.001 to 0.007 μm. Provided that these conditionsare satisfied, it does not cause any inconvenience at all to mix intothe polyester film a rough surface-giving substance well known in theart such as, for example, calcium carbonate, kaolinite, titaniumdioxide, silica or alumina.

It causes no inconvenience to carry out a treatment for securing slipproperties such as, for example, coating or laminating of a polyesterfilm containing a rough surface-giving substance onto the surfacereverse to the surface of the polyester film on which a magnetic layeris formed, in such a range that the resultant coat, laminate or the likeis not transferred onto the surface on which the magnetic layer isformed.

Preparation of a metallic thin membrane magnetic recording medium usingthe above polyester film of this invention can be carried out by amethod known per se, for example by a method disclosed in JapaneseLaid-Open Patent Publication No. 147010/1979 or Japanese Laid-OpenPatent Publication No. 134706/1977, and specifically, a vacuumdeposition method, an ion plating method and a sputtering method canpreferably be used.

Metals forming metallic thin membranes are not particularly limited, butthere can be mentioned cobalt, cobalt-nickel, cobalt-chromium, iron,etc.

The thickness of a metallic thin membrane formed on the surface of thepolyester film is generally 0.01 to 0.6 μm.

The magnetic recording medium of this invention has on the surface ofthe polyester film a magnetic layer, particularly metallic thin membranemagnetic layer which is smooth and excellent in running properties anddurability in preservation, and is a magnetic recording medium,particularly metallic thin membrane magnetic recording medium which hasa characteristic of great decrease of noise and is extremely excellentin noise level, and is excellent in running properties of the magneticsurface, particularly metallic thin membrane surface.

BRIEF DESCRIPTION OF THE DRAWING

The drawing is a schematic drawing of an apparatus to measure acoefficient of dynamic friction for evaluation of film runningproperties. Numbers in the drawing have the following meanings,respectively.

1 feed reel

2 tension controller

3,5,6,8,9,11 free roller

4 tension detector (entrance)

7 fixed pin

10 tension detector (exit)

12 guide roller

13 winding reel

Various characteristics in the present specification are defined ormeasured as follows:

(1) Intrinsic Viscosity [η]

Determined from a value measured at 35° C. in an orthochlorophenolsolvent.

(2) Average Particle Size of Particles

The circle-equivalent diameter of each particle is determined from thephotograph of the particles by a scanning type electron microscope usingan image analysis treatment apparatus (Luzex 500), and the average valueof the circle-equivalent diameters of 100 particles is defined as theaverage particle size.

(3) Number of Protuberances on the Film Surface

Counted by a scanning type electron microscope.

(4) Surface Roughness Ra (Center Line Average)

A chart is depicted on a film sample under conditions of a radius of thestylus being 2 μm, a load of 30 mg, an enlarged magnification of200,000-fold and a cut-off of 0.08 mm according to JIS B 0601 using ahighly accurate surface roughness meter SE-3 FAT produced by KosakaLaboratories Co., Ltd. A part of the film sample is picked up in alength of L from the surface roughness curve thus obtained in thedirection toward a center line, and when the center line of theextracted part is defined as an X axis and an axis in the direction oflongitudinal magnification as a Y axis and the surface roughness curveis expressed by a formula Y=f(x), the value given by the followingformula is expressed as by a μm unit. ##EQU3##

This measurement is carried out on 4 film samples with setting thestandard length to 1.25 mm, and Ra is expressed as the average value.

(5) Film Coefficient of Friction (Film Slipperiness)

A coefficient of static friction (μm) is measured according to ASTMD1894-63 using a slippery measurement apparatus produced by Toyo TesterCo., Ltd., provided that a glass plate is used as the sled plate and aload of 1 kg is applied.

Film slipperiness is judged based on the following criterion:

◯: good (μs under 0.6)

Δ: somewhat inferior (μs 0.6 to 0.8)

X: inferior (μs 0.8 or more)

(6) Running Durability

The attached FIG. 1 is a schematic drawing of an apparatus forevaluation of film running properties. In FIG. 1, 1 represents a feedreel; 2 a tension controller; 3,5,6,8,9 and 11 free rollers; 4 a tensiondetector (entrance); 7 a chromium-plated fixed pin (5 mmo); 10 a tensiondetector (exit); 12 a guide roller; and 13 a winding reel, respectively.

As shown in FIG. 1, under an atmosphere of 20° C. and 60% RH, a film iscontacted with the fixed pin of an outer diameter of 5 mm at an angleθ=(152/180) radian (152°), and moved and rubbed at a speed of 3.3cm/sec. The tension controller 2 is adjusted so that entrance tension(T1) becomes 30 g, the film is run by 10m and rewound, and again runningis repeated. This reciprocation is counted once.

1) Abrasion Resistance

It is observed whether there is a substance deposited on the fixed pinafter 30 times repeated running, and evaluation is made according to thefollowing criterion:

◯: deposit is scarcely observed

Δ: there are signs of some adhesion

X: there are signs of much adhesion

2) Scratch Resistance

The friction state (degree of occurrence of scratch) of the film surfaceafter 30 times repeated running is observed, and evaluated by thefollowing criterion:

◯: scratch is scarcely observed

X: considerable degree of scratch occurs

(7) Electromagnetic Conversion Characteristics

A film is wound along with a cylindrical can having a diameter of 1m, aCo-Ni alloy (containing 20 wt % Ni) is rhombicly vapor deposited to amembrane thickness about 1500 Å at a minimal angle of incidence of 43°in 5×10⁻⁵ (Torr) of oxygen, and thereafter, the film is slitted to abreadth of 8 mm to prepare a magnetic tape. Using this magnetic tape,high density recording characteristics, particularly magnitude of noiselevel and evaluated based on the S/N (dB) ratio at the time of 10 KBPIrecording and reproduction and the lowering rate of the output at thetime of 50 KBPI recording and reproduction against the output at thetime of 10 KBPI recording and reproduction:

S/N (dB) at the time of 10 KBPI recording and reproduction

◯: 45 dB or more

X: under 45 dB

Output lower rate

A=(output at the time of 10 KBPI recording and reproduction)/(output atthe time of 50 KBPI recording and reproduction)

◯: A is under 10

X: A is 10 or more

(8) Tape Running Properties

Picture recording and reproduction are repeated under two kinds ofconditions, namely at ordinary temperature and ordinary humidity, and ata high temperature and a high humidity using a 8 mm, VCR (Video CassetteRecorder) generally on the market, and fluctuation of the image due toturbulence of tape running at that time is observed. The evaluationcriterion is as follows:

◯: running is smooth and there is no fluctuation of reproductionevaluation

X: running becomes late here and there, and fluctuation of reproducedimage occurs

(9) Scuff Resistance (Adhesion)

Evaluation of scuff resistance is carried out by observation of scuffson the thin membrane of a tape after 100 times repeated running underconditions of ordinary temperature and ordinary humidity as well as ahigh temperature and a high humidity. The evaluation criterion is asfollows:

⊚: occurrence of scuffs is scarcely observed on the thin membranesurface of the tape

◯: a little occurrence of extremely weak scuffs is observed on the thinmembrane surface of the tape

X: intense scuffs occur on the thin membrane surface of the tape

In the above, the conditions of ordinary temperature and ordinaryhumidity is 25° C. and 60% RH, and the condition of a high temperatureand a high humidity is 40° C. and 80% RH.

(10) Tape Durability in Preservation

A 0.5% myristic acid/isopropyl alcohol solution is applied onto the tapesurface in a quantity of 10 g/m², the resultant tape is air dried atordinary temperature and held at 60° C. and 90% RH for 24 hours, andthen the degree of deformation of the tape surface is observed andevaluated by a microscope (magnification: 200-fold or 400-fold). Theevaluation criterion is as follows:

◯: the tape surface is not deformed at all even by observation at400-fold

Δ: Although no deformation of the tape surface is found by observationat 200-fold, small deformation is found by observation at 400-fold

X: deformation of the tape surface is found by observation at 200-fold

EXAMPLES

Hereafter, this invention is further described by examples. The "parts"in examples mean weight parts.

Example 1

0.019 part of manganese acetate tetrahydrate and 0.013 part of sodiumacetate trihydrate were charged, together with 100 parts of dimethylterephthalate and 70 parts of ethylene glycol, into a reactor, andtransesterification reaction was carried out with gradual rise of theinner temperature from 145° C. When the conversion oftransesterification became 95%, thereto was added as a stabilizer 0.044part of a glycol solution of a phosphorus compound previously obtainedby reaction of 25 parts of trimethyl phosphate and 75 parts of ethyleneglycol. Further, thereto was added as a polymerization catalyst 0.011part of a liquid (titanium content: 11 weight %) previously obtained bydissolving 0.8 part of trimellitic anhydride in 2.5 parts of ethyleneglycol and adding dropwise thereto 0.65 part of tetrabutyl titanate tocarry out reaction. Then the reaction product was transferred into apolymerization reactor and subjected to polycondensation reaction undera high temperature and vacuum (final inner temperature: 290° C.) toobtain polyethylene terephthalate having an intrinsic viscosity of 0.60.

This polyethylene terephthalate was melt-extruded according to aconventional method, and rapidly cooled to prepare an unstretched filmhaving a thickness of 131 μm. This unstretched film was subjected toconsecutive biaxial stretching of 3.6-fold at 90° C. in the longitudinaldirection and then 3.7-fold at 105° C. in the transverse direction, andfurther, heat-set at 205° C. for 30 seconds to prepare a biaxiallyoriented film having a thickness of 9.8 μm.

In the above, a coating liquid having the following composition wasapplied on the surface (one side) of the monoaxially stretched filmbefore the transverse stretching by a roll coating method.

Composition of the Coating Liquid Applied onto the Film Surface

    ______________________________________    A 0.5 wt % water-dispersion of fine particles                              40.0    parts    consisting of crosslinked acrylic spherical particles    having an average particle size of 0.025 μm coated    with a polyester resin    A 0.5 wt % aqueous solution of polyethylene                              60.0    parts    nonylphenyl ether (NS 208.5 produced by Nippon    Oils and Fats Co., Ltd.)    ______________________________________

The applied quantity was 0.5 g/m² on the basis of wet weight.

In Table 1 are shown characteristics of the obtained polyester film anda magnetic recording medium wherein a magnetic layer was provided on thecoated surface.

When the coated surface of the film was observed by a scanning typeelectron microscope, protuberances not containing fine particlesrespectively were not substantially observed.

Example 2

The procedure of Example 1 was repeated except that a coating liquidhaving the following composition was applied onto the surfaces (A) and(B) of the monoaxially stretched film before the transverse stretchingby a roll coating method. Surface (A) forms the first surface andsurface (B) the second surface.

Composition of the Coating Liquid Applied onto the Film Surface (A)

    ______________________________________    A 0.1 wt % water-dispersion of fine particles                              30.0    parts    consisting of silica fine particles having an average    particle size of 0.04 μm coated with an acrylic    resin    A 0.1 wt % aqueous solution of polyoxyethylene                              70.0    parts    nonylphenyl ether (NS 208.5 produced by Nippon    Oils and Fats Co., Ltd.)    ______________________________________

The applied quantity was 0.12 g/m² on the basis of wet weight.

Composition of the Coating Liquid Applied onto the Film Surface (B)

    ______________________________________    A 2.0 wt % aqueous solution of an acrylicpoly-                              56.7    parts    ester resin (Pesresin SH 551 produced by    Takamatsu Oils and Fats Co., Ltd.)    A 2.0 wt % aqueous solution of a cellulose resin                              24.3    parts    (Methylcellulose SM-15 produced by Shin-Etsu    Chemical Co., Ltd.)    A 2.0 wt % water-dispersion of fine particles of                              9.0     parts    polymethyl methacrylate (Eposter MA produced    by Nippon Shokubai CO., LTD.)    A 2.0 wt % aqueous solution of polyoxyethylene                              10.0    parts    nonylphenyl ether (NS 208.5 produced by Nippon    Oils and Fats Co., Ltd.)    ______________________________________

The applied quantity was 4.2 g/m² on the basis of wet weight.

In Table 1 are shown characteristics of the obtained polyester film anda magnetic recording medium wherein a magnetic layer was provided on thefirst surface of this film.

When the first surface (the coated surface) of film was observed by ascanning type electron microscope, protuberances not containing fineparticles respectively were not substantially observed.

Example 3

The procedure of Example 1 was repeated except that 0.0107 part ofgermanium dioxide was used as a polymerization catalyst to obtainpolyethylene terephthalate (A) having an intrinsic viscosity of 0.60.

0.040 part of manganese acetate tetrahydrate was charged, together with100 parts of dimethyl terephthalate and 70 parts of ethylene glycol,into a reactor, and transesterification reaction was carried out withgradual rise of the inner temperature from 145° C. When the conversionof transesterification became 97%, 0.100 part of a glycol solution of aphosphorus compound previously obtained by reaction of 25 parts oftrimethyl phosphate with 75 parts of ethylene glycol was added as astabilizer, and then 0.045 part of antimony trioxide was added as apolymerization catalyst. Five minutes thereafter, 0.10 parts of calciumcarbonate having an average particle size of 0.35 μm was added, and thenthe reaction product was transferred into a polymerization reactor andsubjected to polycondensation reaction under a high temperature andvacuum (final inner temperature: 290° C.) to obtain polyethyleneterephthalate (B) having an intrinsic viscosity of 0.65.

These polyethylene terephthalates (A) and (B) were melt co-extruded in arate of thickness ratio 8:2 and rapidly cooled to prepare an unstretchedfilm having a thickness of 96 μm. The unstretched film was subjected toconsecutive biaxial stretching of 3.6-fold at 95° C. in the longitudinaldirection and 3.7-fold at 110° C. in the transverse direction, andfurther heat-set at 205° C. for 30 seconds to prepare a biaxiallyoriented film having a thickness of 7.1 μm. In the above, a coatingliquid having the following composition was applied, by a roll coatingmethod, onto the outer surface of polyethylene terephthalate (A) of themonoaxially stretched film before the transverse stretching.

Composition of the Coating Liquid Applied onto the Film Surface

    ______________________________________    A 0.7 wt % water-dispersion of fine particles                              50.0    parts    consisting of silicone tabular particles having an    average particle size of 0.05 μm coated with a    polyurethane resin    A 0.7 wt % aqueous solution of polyoxyethylene                              50.0    parts    nonylphenyl ether (NS 240 produced by Nippon    Oils and Fats Co., Ltd.)    ______________________________________

The applied quantity was 1.0 g/m² on the basis of wet weight.

In Table 1 are shown characteristics of the obtained polyester film anda magnetic recording medium wherein a magnetic layer was provided on theouter surface of polyethylene terephthalate (A) of this film.

When the outer surface (the coated surface) of polyethyleneterephthalate (A) was observed by a scanning type electron microscope,protuberances not containing fine particles respectively were notsubstantially observed.

Example 4

A mixture of 100 parts of bis-β-hydroxyethyl ester of terephthalic acid,65 parts of terephthalic acid and 29 parts of ethylene glycol wassubjected to esterification reaction at temperatures of 210° to 230° C.When the quantity of water formed by the reaction and distilled outbecame 13 parts, reaction was completed and 0.0067 part of titaniumacetate was added per 100 parts of the reaction product. The resultantreaction product was transferred into a polymerization reactor, 0.11part of an ethylene glycol slurry of silicon dioxide having an averageparticle size of 0.015 μm (10 wt % liquid) was added, and the mixturewas subjected to polycondensation reaction under a high temperature andvacuum (final inner temperature: 285° C.) to obtain polyethyleneterephthalate having an intrinsic viscosity of 0.60.

According to conventional methods, this polyethylene terephthalate wasmelt-extruded and rapidly cooled to prepare an unstretched film having athickness of 96 μm. The unstretched film was subjected to consecutivebiaxial stretching of 3.6-fold at 90° C. in the longitudinal directionand 3.7-fold at 105° C. in the transverse direction, and furthersubjected to heat-setting at 205° C. for 30 seconds to prepare abiaxially oriented film having a thickness of 7.1 μm. Thereafter, acoating liquid having the following composition was applied onto thefilm by a roll coating method, and the resultant film was heat treatedfor 10 seconds with hot air of 130° C.

Composition of a Coating Liquid Applied onto the Film Surface

    ______________________________________    Fine particles consisting of polystyrene                              40.0    parts    spherical fine particles having an average particle    size of 0.04 μm coated with a polyvinyl acetate    resin    Xylene                    58.0    parts    Methyl ethyl ketone       2.0     parts    ______________________________________

The applied quantity was 0.2 g/m² on the basis of wet weight.

In Table 1 are shown characteristics of the obtained polyester film anda magnetic recording medium wherein a magnetic layer was provided onthis film.

When the coated surface of the film was observed by a scanning typeelectron microscope, protuberances not containing fine particlesrespectively were not substantially observed.

Example 5

0.021 part of manganese salicylate and 0.005 part of potassium acetatewere added to a mixture of 100 parts of dimethyl2,6-naphthalenedicarboxylate and 60 parts of ethylene glycol, andfurther, 0.007 part of titanium oxalate was added, andtransesterification reaction was carried out with gradual rise oftemperature from 150° C. to 240° C. When the conversion oftransesterification reached 92%, 0.032 parts of a mixed liquidpreviously obtained by reaction of 25 parts of trimethyl phosphate and75 parts of ethylene glycol was added as a stabilizer in a state of aliquid temperature of 140° C., and further, 0.200 part of an ethyleneglycol slurry of silicon dioxide having an average particle size of0.010 μm (10 wt %) was added. Then, the reaction product was transferredinto a polymerization reactor and subjected to polycondensation reactionunder a high temperature and vacuum (final inner temperature: 280° C.)to obtain polyethlene-2,6-naphthalate having an intrinsic viscosity of0.57.

According to conventional methods, this polyethylene-2,6-naphthalate wasmelt-extruded and rapidly cooled to prepare an unstretched film having athickness of 60 μm, and the unstretched film was subjected toconsecutive biaxial stretching of 4.0-fold at 135° C. in thelongitudinal direction and 3.8-fold at 145° C. in the transversedirection and further subjected to heat-setting at 210° C. for 30seconds to prepare a biaxially oriented film having a thickness of 4.0μm. In the above, a coating liquid having the following composition wasapplied onto the surface of the monoaxially stretched film before thetransverse stretching by a roll coating method.

Composition of a Coating Liquid Applied onto the Film Surface

    ______________________________________    A 0.4 wt % water-dispersion of fine particles                              40.0    parts    consisting of tabular titanium oxide fine particles    having an average particle size of 0.03 μm coated    with an epoxy resin    A 0.4 wt % aqueous solution of polyoxyethylene                              60.0    parts    nonylphenyl ether (NS 240 produced by Nippon    Oils and Fats Co., Ltd.)    ______________________________________

The applied quantity was 1.0 g/m² on the basis of wet weight.

In Table 1 are shown characteristics of the resultant polyester film anda magnetic recording medium wherein a magnetic layer was providedthereon.

When the coated surface of the film was observed by a scanning typeelectron microscope, protuberances not containing fine particlesrespectively were not substantially found.

Comparative Example 1

The procedure of Example 1 was repeated except that the coating liquidapplied onto the polyethylene terephthalate film was changed as followsto obtain a film.

Composition of a Coating Liquid Applied onto the Film Surface

    ______________________________________    A 0.03 wt % water-dispersion of fine particles                              40.0    parts    consisting of crosslinked acrylic spherical fine    particles having an average particle size of    0.07 μm coated with a polyester    A 0.03 wt % aqueous solution of polyoxyethylene                              60.0    parts    nonylphenyl ether (NS 208.5 produced by Nippon    Oils and Fats Co., Ltd.)    ______________________________________

The applied quantity was 0.05 g/m² on the basis of wet weight.

In Table 1 are shown characteristics of the resultant polyester film anda magnetic recording medium wherein a magnetic layer was provided on thesurface of this film.

Comparative Example 2

The procedure of Example 1 was repeated except that after the additionof the polymerization catalyst was further added 4.5 parts of a 10 wt %ethylene glycol slurry of calcium carbonate having an average particlesize of 0.7 μm, whereby was obtained polyethylene terephthalate havingan intrinsic viscosity of 0.60.

According to conventional methods, this polyethylene terephthalate wasmelt-extruded and rapidly cooled to prepare an unstretched film having athickness of 131 μm, and then, the unstretched film was subjected toconsecutive biaxial stretching of 3.6-fold at 90° C. in the longitudinaldirection and 3.7-fold at 105° C. in the transverse direction, andfurther subjected to heat-setting at 205° C. for 30 seconds to prepare abiaxially oriented film having a thickness of 9.8 μm. In the above, acoating liquid having the following composition was applied by a rollcoating method onto the surface of the monoaxially stretched film beforethe transverse stretching.

Composition of the Coating Liquid Applied onto the Film Surface

    ______________________________________    A 0.6 wt % water-dispersion of fine particles                              40.0    parts    consisting of crosslinked acrylic spherical particles    having an average particle size of 0.030 μm coated    with a polyester resin    A 0.6 wt % aqueous solution of polyoxyethylene                              60.0    parts    nonylphenyl ether (NS 208.5 produced by Nippon    Oil and Fats Co., Ltd.)    ______________________________________

The applied quantity was 0.5 g/m² on the basis of wet weight.

In Table 1 are shown characteristics of the resultant polyester film anda magnetic recording medium wherein a magnetic layer was provided on thesurface of this film.

Comparative Example 3

The procedure of Example 1 was repeated except that the coating liquidapplied onto the film surface was changed as follows.

Composition of the Coating Liquid Applied onto the Film Surface

    ______________________________________    A 20.0 wt % water-dispersion of fine particles                              50.0    parts    consisting of silica fine particles having an average    particle size of 0.03 μm coated with an acrylic    resin    A 20.0 wt % aqueous solution of polyoxyethylene                              50.0    parts    nonylphenyl ether (NS 208.5 produced by Nippon    Oils and Fats Co., Ltd.)    ______________________________________

The applied quantity was 70 g/m² on the basis of wet weight.

In Table 1 are shown characteristics of the resultant polyester film anda magnetic recording medium wherein a magnetic layer was provided on thesurface of this film.

Comparative Example 4

The procedure of Example 1 was repeated except that 0.20 part of a 10 wt% ethylene glycol slurry of silicon dioxide having an average particlesize of 0.25 μm was further added after the addition of thepolymerization catalyst, whereby was obtained polyethylene terephthalatehaving an intrinsic viscosity of 0.60.

According to conventional methods, this polyethylene terephthalate wasmelt-extruded and rapidly cooled to prepare an unstretched film having athickness of 131 μm, and the unstretched film was subjected toconsecutive biaxial stretching of 3.6-fold at 90° C. in the longitudinaldirection and 3.7-fold at 105° C. in the transverse direction andfurther subjected to heat-setting at 205° C. for 30 seconds to prepare abiaxially oriented film having a thickness of 9.8 μm. In the above, acoating liquid having the following composition was applied onto thesurface of the monoaxially stretched film before the transversestretching.

Composition of the Coating Liquid Applied onto the Film Surface

    ______________________________________    A 1.5 wt % aqueous solution of an acrylicpoly-                              75.0    parts    ester resin (Pesresin SH 551 produced by    Takamatsu Oil and Fat Co., Ltd.)    A 1.5 wt % water-dispersion of silica fine                              10.0    parts    particles having an average particle size    A 1.5 wt % aqueous solution of polyoxyethylene                              15.0    parts    nonylphenyl ether (NS 240 produced by Nippon    Oils and Fats Co., Ltd.)    ______________________________________

The applied quantity was 3.0 g/m² on the basis of wet weight.

In Table 1 are shown characteristics of the resultant polyester film anda magnetic recording medium wherein a magnetic layer was provided on thesurface of this film.

When the coated surface of this film was observed by a scanning typeelectron microscope, a lot of protuberances (length: about 0.03 μm) notcontaining silica fine particles respectively were found in addition toprotuberances containing silica fine particles respectively.

Comparative Example 5

The procedure of Example 1 was repeated except that the coating liquidapplied onto the film surface was changed as follows.

Composition of the Coating Liquid Applied onto the Film Surface

    ______________________________________    A 5.0 wt % water-dispersion of fine particles                              70.0    parts    consisting of calcium carbonate particles having an    average particle size of 0.25 μm with an acrylic    resin    A 5.0 wt % aqueous solution of polyoxyethylene                              30.0    parts    nonylphenyl ether (NS 208.5 produced by Nippon    Oils and Fats Co., Ltd.)    ______________________________________

The applied quantity was 30 g/m² on the basis of wet weight.

In Table 1 are shown characteristics of the resultant polyester film anda magnetic recording medium wherein a magnetic layer was provided on thesurface of this film.

    __________________________________________________________________________           Average particle           size of fine                    Number of pro-                                  Film                                      Running durability           particles as                    tuberances on                            Surface                                  friction                                      Chipping                                           Scratch           nuclei   the film surface                            roughness                                  coeffi-                                      proper-                                           resist-           (μm)  (g/mm.sup.2)                            (μm)                                  cient                                      ties ance    __________________________________________________________________________    Example 1           0.025    5.0 × 10.sup.7                            0.0013                                  ◯                                      ◯                                           ◯    Example 2           0.04     8.0 × 10.sup.5                            0.0012                                  ◯                                      ◯                                           ◯    Example 3           0.05     4.0 × 10.sup.5                            0.0012                                  ◯                                      ◯                                           ◯    Example 4           0.04     4.0 × 10.sup.7                            0.0035                                  ◯                                      ◯                                           ◯    Example 5           0.03     8.0 × 10.sup.6                            0.0050                                  ◯                                      ◯                                           ◯    Comparative           0.07     3.0 × 10.sup.3                            0.0013                                  X   X    ◯    Example 1    Comparative           0.03     2.0 × 10.sup.7                            0.040 ◯                                      ◯                                           ◯    Example 2    Comparative           0.03     2.0 × 10.sup.11                            0.0050                                  ◯                                      X    ◯    Example 3    Comparative           0.03     9.0 × 10.sup.7                            0.0080                                  ◯                                      X    ◯    Example 4    Comparative           0.25     2.0 × 10.sup.6                            0.080 ◯                                      X    ◯    Example 5    __________________________________________________________________________                         Magnetic recording medium                            Rate of                                 Running                                      Scuff                                          Durability                            Output                                 proper-                                      resist-                                          in preser-                         S/N                            lowering                                 ties ance                                          vation    __________________________________________________________________________                  Example 1                         ◯                            ◯                                 ◯                                      ⊚                                          ◯                  Example 2                         ◯                            ◯                                 ◯                                      ◯                                          ◯                  Example 3                         ◯                            ◯                                 ◯                                      ⊚                                          ◯                  Example 4                         ◯                            ◯                                 ◯                                      ⊚                                          ◯                  Example 5                         ◯                            ◯                                 ◯                                      ⊚                                          ◯                  Comparative                         ◯                            ◯                                 X    X   ◯                  Example 1                  Comparative                         X  X    ◯                                      ⊚                                          ◯                  Example 2                  Comparative                         X  ◯                                 ◯                                      ⊚                                          Δ                  Example 3                  Comparative                         X  ◯                                 ◯                                      ◯                                          X                  Example 4                  Comparative                         X  X    ◯                                      X   X                  Example 5    __________________________________________________________________________

We claim:
 1. A polyester film for magnetic recording media having fineprotuberances on the film surface of one or both sides of the polyesterfilm, the protuberances comprising fine particles coated with a binderresin with which the protuberances are bound to the film surface,wherein:(i) the binder resin is substantially confined to areas of thefilm surface where the fine protuberances exist such that other areas ofthe film surface are substantially free of binder resin; (ii) the fineprotuberances are bound to the film surface at a density of 1.0×10⁴ to1.0×10⁸ per mm² and have an average height of 0.15 μm or less; and (iii)the polyester film possesses a centerline average surface roughness (Ra)in the range of 0.001 to 0.01 μm.
 2. The film of claim 1 wherein theaverage height of the fine protuberances on the film surface is 0.1 μmor less.
 3. The film of claim 1 wherein the fine protuberances are boundto the film surface at a density of 1.0×10⁵ to 8×10⁷ per mm².
 4. Thefilm of claim 1 wherein the particles are such that the relativestandard deviation value of their particle sizes is 0.5 or less.
 5. Thefilm of claim 1 wherein the particles have an average particle size of0.1 μm or less.
 6. The film of claim 1 wherein the centerline averagesurface roughness (Ra) of the film surface is in the range of 0.001 to0.007 μm.
 7. The film of claim 1 wherein the binder resin has asoftening point of 80° to 240° C.
 8. A magnetic recording medium whichcomprises the polyester film of claim 1 having a ferromagnetic metallicmembrane formed thereon and over the protuberances on the side or sidesof the polyester film on which said protuberances are formed.
 9. Themagnetic recording medium of claim 8 wherein the ferromagnetic metallicmembrane has a thickness of 0.01 to 0.6 μm.
 10. The magnetic recordingmedium of claim 8 wherein the ferromagnetic metallic membrane iscomposed of cobalt or a cobalt alloy.